Capaday Charles
Department of Anatomy and Physiology, Université Laval, Québec City, Canada.
Exp Brain Res. 2002 Mar;143(1):67-77. doi: 10.1007/s00221-001-0970-z. Epub 2001 Dec 21.
It has been suggested that balancing excitatory and inhibitory conductance levels can control the firing rate gain of single neurons, defined as the slope of the relation between discharge frequency and excitatory conductance. According to this view the increase in firing rate produced by an input pathway can be controlled independently of the ongoing firing rate by adjusting the mixture of excitatory and inhibitory conductances produced by other pathways converging onto the neuron. These conclusions were derived from a simple RC-neuron model with no active conductances, or firing threshold mechanism. The analysis of that model considered only the subthreshold behaviour and did not consider the relation between total trans-membrane conductance and firing rate. Similar conclusions were also derived from a simple parallel conductance based model. In this paper I consider, as an example of a repetitively firing neuron, a generic model of cat lumbar alpha-motoneurons with excitatory and inhibitory inputs and a second independent excitatory pathway. The excitatory and inhibitory inputs can be thought of as central descending controls while the second excitatory pathway may represent, for example, the monosynaptic Ia-afferent pathway. I have re-examined the possibility that the firing rate gain of the "afferent" pathway can be controlled independently of the ongoing firing rate by balancing the excitatory and inhibitory conductances activated by the descending inputs. The steady state firing rate of the model motoneuron increased nearly linearly with the excitatory current, as it does in real motoneurons (primary firing range). The model motoneuron also showed a secondary firing range, whose slope was steeper than in primary range. The firing rate gain was measured by increasing the conductance of the "afferent" pathway. The firing rate gain (in the primary and secondary firing range) of the "afferent" pathway was found to be the same regardless of the particular mixture of excitatory and inhibitory conductances acting to produce the ongoing firing rate. This result was obtained for a single-compartment model, as well as for a two-compartment model consisting of an active somatic compartment and a dendritic compartment containing an L-type calcium conductance. Put simply, the firing rate gain of an input to a neuron cannot be controlled by balancing excitatory and inhibitory conductances produced by other independent input pathways, or by the spatial distribution of excitation and inhibition across the neuron. Three potential ways of controlling the firing rate gain are presented in the "Discussion". Firing rate gain can be controlled by actions at the presynaptic terminal, by inhibitory feedback, which is a function of the neuron's firing rate, or by neuromodulator substances that affect intrinsic inward or outward currents.
有人提出，平衡兴奋性和抑制性电导水平可以控制单个神经元的放电率增益，放电率增益定义为放电频率与兴奋性电导之间关系的斜率。按照这种观点，通过调整汇聚到神经元上的其他通路产生的兴奋性和抑制性电导的混合比例，由输入通路产生的放电率增加可以独立于当前的放电率进行控制。这些结论源自一个没有主动电导或放电阈值机制的简单RC神经元模型。对该模型的分析仅考虑了阈下行为，没有考虑总跨膜电导与放电率之间的关系。类似的结论也源自一个基于简单并联电导的模型。在本文中，作为一个重复放电神经元的示例，我考虑了一个具有兴奋性和抑制性输入以及第二条独立兴奋性通路的猫腰段α运动神经元的通用模型。兴奋性和抑制性输入可以被视为中枢下行控制，而第二条兴奋性通路例如可以代表单突触Ia传入通路。我重新审视了通过平衡下行输入激活的兴奋性和抑制性电导，“传入”通路的放电率增益能否独立于当前放电率进行控制的可能性。模型运动神经元的稳态放电率随兴奋性电流几乎呈线性增加，就像在真实运动神经元中一样（初级放电范围）。模型运动神经元还显示出一个次级放电范围，其斜率比初级范围更陡。通过增加“传入”通路的电导来测量放电率增益。结果发现，无论用于产生当前放电率的兴奋性和抑制性电导的具体混合比例如何，“传入”通路的放电率增益（在初级和次级放电范围内）都是相同的。对于单室模型以及由一个有源胞体室和一个含有L型钙电导的树突室组成的双室模型，都得到了这一结果。简单地说，神经元输入的放电率增益不能通过平衡其他独立输入通路产生的兴奋性和抑制性电导，或者通过神经元上兴奋和抑制的空间分布来控制。“讨论”部分提出了三种控制放电率增益的潜在方式。放电率增益可以通过突触前终末的作用、作为神经元放电率函数的抑制性反馈或者通过影响内在内向或外向电流的神经调质物质来控制。
